This invention relates to a mast staging cushion apparatus for use with lift tracks.
Conventional lift trucks are industrial vehicles for cargo handling which have masts for raising and lowering a load. The masts telescope to achieve desired lifting heights and maintain a low collapsed profile. A common telescopic mast uses a fixed mast section (first stage) and one, two, or three additional moveable mast sections (second, third and fourth stage). A carriage assembly supporting the load carrying forks and/or an operator's platform mounted on the telescoping mast may be raised from floor level to the maximum lift height of the mast. Two distinct load movements may occur: elevation of the load without extension of the moveable mast section(s) (free lift) and above this height, elevation of the load with corresponding upward movement of the moveable mast section(s) (staged lift). In sequence, the telescoping sections are collapsed to the height of the fixed section and the carriage is raised from floor level until reaching the top of the telescopic section in which it is mounted. Further elevation of the carriage then causes the moveable section(s) to telescope upwardly until the carriage reaches maximum lift height. Staged lift and free lift hydraulic cylinders and lifting chains are used to move the carriage and the mast sections in the proper sequence.
One problem with the telescopic mast structure occurs during mast staging (i.e. at the transition between free lift and staged lift movements). When staging occurs, sudden start (during lift) or stopping (during lowering) of the moveable mast sections creates impact loads (shocks) and rapid changes in acceleration (jerks) that can be a source of noise and discomfort for the operator. Historically, various mechanical cushioning devices or hydraulic cylinder cushions have been used to reduce these effects during staging. These devices are marginally effective and typically work for raising only or lowering only but not for both. Common designs use a mechanical bumper or spring for raising and a hydraulic cylinder cushion for lowering. Also, the effectiveness of these types of cylinder cushions are influenced by changing loads or operating speeds which is not desirable.